 ## Fluids

Topics relate to Fluent, CFX, Turbogrid and more

• James
Subscriber

Hello!

I am looking to solve the following transport equation: it's a fluid phase mass balance used in packed beds. Anyway, since I will be defining a custom source term, I suppose fluent would use the transport equation for the UDS, in that case I noticed that having a custom UDS unlocks the "UDS diffusivity" option in the materials tab, but it's units are kg/ms, however, the units of the diffusion coefficient in my transport term are m2/s. How do I define a proper diffusivity coefficient in my case?

Moreover, I have a porous zone in my model and I need to change its density for \rho_p in my transport equation, is that possible?

Thank you for the help!

• James
Subscriber

Okay so from what I found out, since fluent solves density*diffusion, the units (kg/m3 * m2/s) result in kg/ms. So I suppose when you are defining diffusivity in the “materials” tab, you multiply your diffusivity with the fluid density.

My concern remains with the source term, will fluent automatically multiply density of the fluid or do I have to multiply is manually.

• Rob
Ansys Employee

Scalars don't have a density value as it's not a species/phase/real. So you need to (generally) divide the diffusivity in the model by the fluid density.

The source term is whatever you want to add. For fluid mass you're adding some kg/m3/s but for a scalar it's a little different.

• James
Subscriber

Hello Rob,

My apologies as this is an "old" thread now. I am still quite confused. Perhaps I should add more information. The units of C is mg/L, so while defining the boundary condition for C, I used the value of "specified value" that is in mg/L. So if that's the case, when fluent multiplies the fluid density, the dimension of the unsteady term, for example, will be kgmg/Lsm3. The convective term I suppose will follow, and so in this case I should supply the value of diffusivity in kg/ms right? 